The present invention relates to a control loop circuit apparatus of the type used to compensate modulation amplitude control of a laser device capable of transmitting a broadband signal, for the purpose of controlling an extinction ratio of a semiconductor laser device. The present invention also relates to a method of compensating modulation amplitude control.
In an optical communications network, a semiconductor laser device can be employed in a transmitter for transmission of information to a receiver coupled to the network. When driving the laser device, it is important to be able to control a parameter of the laser device known as the extinction ratio. The extinction ratio is a ratio of a first power level of electromagnetic radiation, for example visible light, emitted by the laser device to communicate a LOGIC 1, to a second power level of light, emitted by the laser device to communicate a LOGIC 0.
A control loop is usually provided comprising a feedback path including, inter alia, a photodiode to receive light emitted from a back facet of the laser device. The photodiode generates an electrical feedback current signal that is used to control modulation amplitude, and hence the extinction ratio, of the laser device. The feedback current signal generated by the photodiode corresponds to the information transmitted, for example a bit stream of data, by the laser device. The frequency of the feedback current signal will depend upon the frequency of the bit stream and the bandwidth of the feedback path. By way of explanation, it should be understood that the frequency of the bit stream transmitted by the laser device will depend upon the bit pattern constituting the data being transmitted. The frequency content of actual data can be approximated by a pseudo-random bit stream and so the data can contain a broad range of frequencies over a period of time. The lowest frequency of the bit stream corresponds to a maximum number of consecutive LOGIC 1s or LOGIC 0s in the data stream, the highest frequency of the data stream corresponding to the actual data rate of the data stream.
When the highest frequency in the data stream is greater than the bandwidth of the feedback path, the current feedback signal becomes distorted. Bandwidth of the photodiode is a major factor that contributes to the bandwidth of the feedback path, but also parasitic circuit track capacitance.
In relation to the speed of the photodiode, the problem caused by slow response of the photodiode can be best understood by reference to an eye diagram. In this respect, FIG. 1 is a schematic representation of an eye diagram 100 of the type generated in a known way using an oscilloscope. The eye diagram 100 comprises a continuous trace of the pseudo-random bit stream plotted with infinite persistence. A maximum level 102 of the trace corresponds to a maximum amplitude reached by the feedback signal as a result of the bit stream comprising a block of a sufficient number of consecutive LOGIC 1s to allow the feedback signal to climb to the maximum amplitude. Similarly, the minimum level 104 of the trace corresponds to a minimum amplitude attained by the feedback signal as a result of the bit stream comprising a block of a sufficient number of consecutive LOGIC 0s to allow the feedback signal to fall to the minimum amplitude. In contrast, a peak 106 of the trace corresponds to a peak amplitude reached by the feedback signal as a result of a first part of the bit stream containing data having a frequency greater than the bandwidth of the feedback path. Similarly, a trough 108 of the trace corresponds to a trough amplitude reached by the feedback signal as a result of a second part of the bit stream containing data having a frequency greater than the bandwidth of the feedback path.
In order to measure a maximum amplitude of the feedback signal, a central zone 110 is identified by the control loop. However, if the peak 106 or the trough 108 of the trace is measured, the measured amplitude will be an incorrect reflection of the actual amplitude of the light received by the photodiode.
Consequently, it is harder to correctly compensate the modulation amplitude of the laser device when the current feedback signal contains data at frequencies above the bandwidth of the feedback path, because the extinction ratio of the laser device may be correct, but the feedback path may not operate sufficiently rapidly to be able to generate the maximum or minimum amplitudes of the current feedback signal when the frequency of the current feedback signal is high. If the extinction ratio of the laser device is incorrectly measured, the modulation amplitude of the laser device will be incorrectly set. This results in an input of the laser device dropping below a lasing threshold corresponding to a LOGIC 0 in response to an input signal corresponding to the LOGIC 0.
Known techniques for setting the modulation power of the laser diode have employed an analogue or digital signal processor using temperature or bias current of the laser device as a reference signal. These techniques are not particularly attractive due to their reliance upon accurate estimates of performance characteristics of the laser device. Another technique involves superimposing a low frequency signal onto an output signal generated by the photodiode. Although this technique eliminates the need for a very high speed feedback loop, the output signal contains noise as a result of the superimposition.
According to a first aspect of the present invention, there is provided a control loop circuit apparatus for compensating modulation amplitude control of a laser device, the apparatus comprising: means for receiving a first signal for transmission to a receiver; means for receiving a second signal generated by a sensor for monitoring the laser device, the second signal corresponding to the first signal; means for simulating signal generation arranged to use the first signal to generate a simulated signal corresponding to the second signal; a processing unit arranged to receive the second signal and the simulated signal, and arranged to generate an output signal for compensating the modulation amplitude control of the laser device, the output signal constituting an indication of a difference between the second signal and the simulated signal.
Preferably, the processing unit comprises a first normalisation unit for adapting the second signal to a predetermined level and/or a predetermined format.
Preferably, the processing unit comprises a second normalisation unit for adapting the simulated signal to the predetermined level and/or predetermined format.
Preferably, the sensor has a frequency band associated therewith, and the means for simulating signal generation comprises a filter having substantially a same frequency band as at least the frequency band of the sensor.
Preferably the means for simulating signal generation comprises a first amplifier.
Preferably, the apparatus further comprising a second amplifier for amplifying the second signal.
Preferably, the processing unit comprises a differential subtractor.
Preferably, the first normalisation unit is a rectifier or a peak detector, and the second normalisation unit is a rectifier or a peak detector.
According to a second aspect of the present invention, there is provided a transmitter circuit for an optical transceiver unit, the circuit comprising the circuit apparatus as set forth above in accordance with the first aspect of the present invention.
According to a third aspect of the present invention, there is provided an optical communications network comprising the circuit apparatus as set forth above in accordance with the first aspect of the present invention.
According to a fourth aspect of the present invention, there is provided a method of compensating modulation amplitude control of a laser device, the method comprising the steps of: receiving a first signal for transmission to a receiver; receiving a second signal generated by a sensor for monitoring the laser device, the second signal corresponding to the first signal; using the first signal to generate a simulated signal corresponding to the second signal; receiving the second signal and the simulated signal, and generating an output signal for compensating the modulation amplitude control of the laser device, the output signal constituting an indication of a difference between the second signal and the simulated signal.
According to a fifth aspect of the present invention, there is provided a control loop circuit apparatus for compensating modulation amplitude control of a laser device, the apparatus comprising: a first input for receiving a first signal for transmission to a receiver; a second input for receiving a second signal generated by a sensor for monitoring the laser device, the second signal corresponding to the first signal; a simulation arrangement for using the first input signal to generate a simulated signal corresponding to the second signal; a processing arrangement for receiving the second signal and the simulated signal, and arranged to generate an output signal for compensating the modulation amplitude control of the laser device, the output signal constituting an indication of a difference between the second signal and the simulated signal.
It is thus possible, by filtering the first signal to the same bandwidth as the sensor, to generate a reference signal that changes in response to the first signal, thereby eliminating false identification of high speed data. It is also possible to provide an apparatus and method that permits accurate control of the modulation amplitude, and hence the extinction ratio, of the laser device, even when the frequency of the second signal is higher than the bandwidth of the feedback path.